I don't think you can put a plug on romex; it isn't approved for that use.
As a practical matter though, what is going to happen?
To be dangerous you would have to have a overload on the 14 that isn't
enough to trip the breaker; yet is enough to start a fire.
While not impossible, it is most unlikely; especially if the the circuit is
dedicated to your little fan.
Its a judgement call.
The one time I was in a similar situation I changed the breaker, since there
was no need for 20a.
That is the only violation. Without trying to start a flame war I
would point out that on a dedicated motor circuit #14 can be used at
it's 20a rating and if the motor has suplimental overload protection
the breaker could be up to 40a. (1 HP single phase 120v)
That is a question that is usually on the inspector test.
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